[lxc-devel] Report correct filesystem usage / limits on BTRFS subvolumes with quota

[Qu Wenruo]

On 2018年07月31日 21:49, Thomas Leister wrote:
> Dear David,
> hello everyone,
> 
> during a recent project of mine involving LXD and BTRFS I found out that
> quotas on BTRFS subvolumes are enforced, but file system usage and
> limits set via quotas are not reported correctly in LXC containers.
> 
> I've found this discussion regarding my problem:
> https://github.com/lxc/lxd/issues/2180

That's not the expected usage of btrfs qgroup/quota.

Quota only accounts how many bytes are used exclusively or shared
between subvolumes at extent level.

> 
> There was already a proposal to introduce subvolume quota support some
> time ago:
> https://marc.info/?l=linux-btrfs&m=147576434114415&w=2

It's in fact impossible if I didn't miss something.

There are several technical problems in the proposal:

1) Multi-level qgroups
   The real limit is limited by all related qgroups, including higher
   level qgroup.
   Such design makes it pretty hard to calculation the real limit.

2) Different limitations on exclusive/shared bytes
   Btrfs can set different limit on exclusive/shared bytes, further
   complicating the problem.

3) Btrfs quota only accounts data/metadata used by the subvolume
   It lacks all the shared trees (mentioned below), and in fact such
   shared tree can be pretty large (especially for extent tree and csum
   tree).
   Only accounting quota limit would hit real ENOSPC easily IMHO.

> 
> @David as I've seen your response on that topic on the mailing list,
> maybe you can tell me if there are any plans to support correct
> subvolume quota reporting e.g. for "df -h" calls from within a
> container? Maybe there's already something on your / SUSE's roadmap? :-)
> 
> As more and more container environments spin up these days, there might
> be a growing demand on that :-) Personally I'd really appreciate if I
> could read the current file system usage and limit from within a
> container using BTRFS as storage backend.

For current btrfs design, I think it's skeptical to implement such design.
The main problem here is, btrfs doesn't do the full LVM work. (unlike
ZFS IIRC)
It doesn't really manage multiple volumes, that's why it's called
subvolume in btrfs.
A subvolume is not a fully usable fs, it's just a subset of a full fs.
It relies on all the other trees (root tree, extent tree, chunk tree,
csum tree, and quota tree in this case) to do all the work.
Thus it's pretty hard to implement such special purposed df call.

On the other hand, isn't easier to implement special interface for
container to get real disk usage/limit other than using the old vanilla
df interface?

Thanks,
Qu

> 
> Best regards,
> Thomas
> 
> 
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